Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is a static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to a high voltage electrical power system and a DC side connected to a temporary electric power storage means such as capacitors. The STATCOM can supply reactive power to, or absorb reactive power from, the electrical power system. In another apparatus, in the following denoted power compensator, a dc power source is connected to the STATCOM in order to perform also active power compensation. The construction may be used e.g. as a spinning reserve and for compensating for fluctuating energy levels in the power system.
If the dc power source is a high voltage battery a large number of battery cells have to be connected in series to match the dc voltage of the electrical power system. The dc power source includes a number of series- and/or parallel-connected battery cells, each having a voltage of approximately 3-4 V, arranged in battery modules. One or more battery modules are in turn parallel-connected to form a battery unit and several battery units may be series-connected to form a battery stack. A plurality of battery stacks can be series-connected to form a battery string. Moreover, to obtain a desired amount of electric energy (duration of active power) of the dc power source, a number of battery strings could be connected in parallel.
As mentioned above the battery unit includes one or more battery modules connected in parallel. The maximum amount of energy stored in the high voltage battery will be proportional to the number of parallel-connected battery modules. However, even if a single battery module would satisfy the requirements on the amount of stored energy, such a design would not be possible for power compensator availability reasons. With only a single battery module in the battery unit a failure will result in an emergency shut-down of the entire power compensator. With two modules in parallel per battery unit a single failure does not require an immediate shut-down of the power compensator. However, the probability of a failure in any of the modules will increase in proportion to the number of battery modules in the dc power source. Since the number of battery modules in the dc power source is very large there will be a clear risk of a battery module failure between the scheduled service and maintenance intervals. When a module failure occurs in a battery unit there is only one non-failed module left in said battery unit and the power compensator has to be shut-down due to the increased risk of an emergency shut-down of the power compensator as described above. Accordingly, to avoid frequent emergency shut-downs of the power compensator and to secure a continuous provision of necessary energy at least three modules in parallel are needed regardless of the actual requirements of the amount of stored energy, which consequently limits the flexibility of the system for different energy storage needs.
Another problem with the mentioned high voltage battery is the capacity reduction caused by a single battery module failure. If, for example, three battery modules are connected in parallel a failure in a single module in the whole string of possibly more than one hundred battery units in series would lead to a capacity reduction of 33%. The corresponding capacity reduction for four or five modules in parallel is 25% and 20% respectively. A solution to this problem is to add a redundant module in each battery unit. This however is a very costly solution. Alternatively, another battery string could be connected in parallel with the existing string but this would increase the cost and size of the power compensator even further.
Another problem with the high voltage battery described above is that the complete battery string would be unavailable during service and maintenance.